Page 279 - Electric Drives and Electromechanical Systems

276 Electric Drives and Electromechanical Systems

time. If such a collision does occur, the transmission is immediately terminated,
and the sender waits for a short time before retrying the transmission. A different
approach is to use a token passing bus networks, where each node on the network
gets its turn (token) in a pre-assigned sequence and can transmit only at that time.
In a star network topology, Fig. 11.2B, each link connects only two nodes, with
some nodes acting as message centres or hubs, accepting and relaying messages to
other nodes. Due to the computing load, the network hub may not have any
computing time left for other functions.
In ring networks, Fig. 11.2C, messages are passed from one node to the next, in one
direction, around the ring. The computer that is communicating passes control to
the next computer in the ring when it has finished its communication. Computers
waiting to originate communications must wait until they receive control.

11.1.3 OSI and TCP/IP model

To ensure correct communication, both ends of the link need to adopt a common
protocol. The protocol specifies such factors as speed, data coding method, error
checking, and handshaking requirements, and implies that the form of the data must be
useful at both ends of the communication.
To define standards, use is made of the Open Systems Interconnection (OSI) model.
The OSIs seven layers break up communication services into four application service
layers and three network service layers, Fig. 11.3, summarised below:
Layer 1 - Physical: Encodes and physically transfer messages. The repeater node
discussed in Section 11.1.2 belongs to this layer as they do not change any aspect
of the message.
Layer 2 - Data link: Breaks up and reassembles messages, together with error
detection and correction at the bit level. The bridge mode discussed in Section
11.1.2 belongs to this layer as a bridge connects two networks, as procedure and

FIG. 11.3 The relationship between the OSI and TCP/IP network models. In the case of TCP/IP, hardware to
considered to be outside the model's specification.